The present invention relates to inorganic or possibly organically modified particles which can be subjected to targeted leaching out of certain cations and can thus be used as inorganic components in glass ionomer cements. The invention further relates to novel ways of producing such particles, known as ionomers, and to the cements which can be produced therewith.
For the purposes of the present invention, the term xe2x80x9cionomer particlexe2x80x9d refers to inorganic particles which in combination with a preferably acid-containing matrix can be used in a very versatile way as cements (self-curing, light-curable, etc.). For a cement formation reaction to be able to take place at all, these particles have to have a defined or targeted instability, i.e. when combined with water in the presence of a partner with which they are to combine they have to release metal ions which lead to a curing reaction in the partner substance. The composition ranges in which this targeted instability occurs are known to those skilled in the art or can easily be determined, see, for example, the phase diagrams of many systems in xe2x80x9cAlumosilicate Glasses for Polyelectrolyte Cementsxe2x80x9d, A. D. Wilson et al., Ind. Eng. Chem. Prod. Res. Dev. 1980, (19) 263-270, or xe2x80x9cGlass-Ionomer Cementsxe2x80x9d, A. D. Wilson et al., 1988, Quintessence Publishing Co., Inc., Chapter 2 (p. 21 ff.).
The basic makeup of the usually vitreous ionomers usually comprises the ternary system silicon dioxide-aluminum oxide-calcium oxide. Melting these components together gives particles which undergo a two-stage reaction in the presence of, for example, polyalkenoic acids. Here, calcium ions are firstly leached from the glass composite by the attack of protons of the polyalkenoic acids and are complexed by the carboxylate groups of the polyalkenoic acids in an unstable phase or primary curing. Secondary curing then leads to a stable phase in which aluminum cations now also migrate from the glass ionomer. Hydration of the polysalts occurs and aluminum polyalkenoates are formed. At the same time, the outer shell of the aluminum silicate glasses is dissolved by proton attack to form orthosilicic acid. During the further course of the reaction, this orthosilicic acid condenses to form silica gel; a gel layer results.
Some of the reactions described are very slow; however, the reaction can be accelerated by addition of fluoride, for example in the form of fluorspar. Hydroxycarboxylic acids, e.g. tartaric acid, can be added as regulators; they lengthen the processing time and shorten the curing time. Further additives such as aluminum phosphate, cryolite or aluminum trifluoride are known as optimizing processing aids.
The known ionomer particles are obtained by melting together the respective starting compounds (mainly oxides) . Their microstructure is complex. The milling process to which the fused glass ionomer is subjected in order to obtain the desired particles promotes the formation of sharp-edged, nonspherical particles. This makes the resulting abrasion resistance of the ionomer unsatisfactory. The particles formed have a broad particle size distribution and are relatively large; they usually have a diameter far above three microns. They have to be subjected to a complicated classification process in order to be obtained in a size distribution which is still only acceptable to a degree. Apart from the high expenditure of effort, this means a high loss of material and thus extremely poor yields (even when using a plurality of screening steps or air classification steps, a distribution over less than at least one power of ten is not achieved, if only because of the unfavorable geometry) . As a result of the fusion procedure, the aluminum silicate matrix is frequently not homogeneous. Thus, for example, fluorides are embedded in the form of droplets rich in calcium fluoride.
Classical glass ionomer cements having purely inorganic curing, light-curable glass ionomer cements (with additional organic polymerizable components) and compomers (the term is derived from the contraction of the expressions composite and ionomer and is used to refer to cements in which the carboxyl group used is bound to the same molecule which also bears a crosslinkable double bond, see, for example, xe2x80x9cGlasionomers, The next Generationxe2x80x9d, Proc. of the 2nd Int. Symp. on Glass Ionomers, 1994, p. 13 ff.), are frequently used as filling material, especially in dentistry. Partners employed for curing (cement formation) are usually the polyalkenoic acids mentioned. Advantages of these materials are: little or no shrinkage through to an expansion caused by the ionomer reaction as a result of water uptake, presence of fluorides and phosphates desired, good bond with the tooth tissue due to the acid groups in the matrix. However, use in dentistry would also require excellent mechanical properties, a favorable abrasion performance (e.g. during chewing) and good polishability of the fillings. These requirements are not, however, met by the cements mentioned owing to the size and shape of the ionomer particles. Thus, the sharp edges, the size and asymmetry of the particles result in those in the positions near the surface being torn from the cement composite during chewing or polishing, which increases abrasion and makes it virtually impossible to achieve a smooth surface. Additional disadvantages are the lack of ability to be adapted to special problems such as X-ray opacity or specific requirements in respect of transparency, e.g. index of refraction or the like.
It is therefore an object of the present invention to provide ionomer particles of the type mentioned at the outset which do not have the abovementioned disadvantages and in combination with any, preferably acid-modified matrix systems lead to cements which are mechanically more stable than the known particles.
This object is achieved by the provision of ionomer particles which have a spherical or approximately spherical shape.
The ionomer particles are either purely inorganic particles, or they can be organically modified.
The novel ionomer particles preferably have a diameter smaller than that which is presently customary. The particle size can be set, for example, in the range from 5 nm to 50 xcexcm. This can be achieved using various, simple methods, which is explained in more detail further below.
The spherical ionomer particles of the present invention have an inner region and also an outer region which comprises silicon ions and whose cations comprise (a) at least one element which in siliceous compounds can occupy lattice sites of silicon to produce a negative charge excess and (b) at least one element which can compensate the negative charge excess and is selected from among elements of main groups I and II and elements which can occur in divalent form. The cations of group (b) serve to effect the primary curing in the unstable phase, while those of the group (a) serve to effect secondary curing and the formation of the stable phase.
The expressions xe2x80x9csilicon ionsxe2x80x9d and xe2x80x9ccationsxe2x80x9d indicate that the elements concerned are present in bound form, but are not intended to rule out their incorporation in structures having some degree of covalent bonding.
The particles can further comprise appropriate additives, for example phosphate (e.g. as aluminum or calcium phosphate) or fluoride (e.g. as NaF, CaF2 or AlF3).
The spherical ionomer particles preferably contain cluster-like, silicate-containing regions. In a further preferred embodiment of the invention, the ionomer particles are entirely homogeneous. In a third preferred embodiment, they consist of an inner region or core which can have a composition different from that of the outer region (the xe2x80x9cshellxe2x80x9d), in which case the shell can consist of one or more layers. However, it is in each case essential for the outer region, i.e. at least the outermost layer, to have the abovementioned composition. If the structure with an inner region different from the outer region is chosen, this can, if desired, be inert toward leaching and serve as carrier for further properties.
In addition, the surface of the particles can be silanized, which makes incorporation into some matrices easier. Silanization can be carried out by known methods, either in-situ or subsequently depending on the method employed.
The particles can be completely solid (dense) or may have a porous structure.
The particle size can vary as a function of production conditions; it can, if desired, be set within a narrow range. Preference is given to providing relatively small particles, since these have more surface area. As a result, the reactivity is increased and thus the curing of the cement is accelerated or improved. A further advantage of smaller particles is improved translucence of the cement formed. Examples of particle sizes are, for example, from 20 nm to 20 xcexcm or from 0.5 xcexcm to 50 xcexcm; the particle size chosen in each case can then be achieved in a very narrow distribution range which can be significantly less than one power of ten. Smaller particles, e.g. in the range from 50 nm to 1 or 2 xcexcm, are particularly suitable for dental fillings. Apart from the above-described advantages, small particles also allow a particularly high proportion of ionomer to be incorporated. In order to be able to obtain a very particularly high proportion of ionomer in the cement, a specific embodiment of the invention provides a mixture of two or three lots of ionomer particles which each have a defined narrow size distribution whose size ratio to the other lots is such that the smaller particles fit into the gaps of a conceptual close packing of spheres of the larger particles and, if present, the still very much smaller particles fit into gaps in the resulting packed arrangement. This embodiment, too, is particularly suitable for dental fillings because a high ionomer content in the cement can be achieved. However, the invention provides not only relatively small particles but also relatively large particles, whether as largest fraction of a size mixture as described above or for use of the cements in other medical or non-medical fields (e.g. as bone replacement or as adhesive).
The spherical ionomer particles can be produced by various methods. Here, a dispersion comprising organic components is formed, and a controlled hydrolysis and condensation proceeds in this dispersion. The expression xe2x80x9cdispersionxe2x80x9d is used here although it is also possible for genuine solutions, suspensions or emulsions to be obtained or to be formed in particular stages of the hydrolytic condensation. Sol and gel formation processes are also encompassed by the expression (e.g. the disperse phase of a dispersion or emulsion can gel). It is therefore to be understood in appropriately broad terms. The dispersion can be converted into spherical particles in various ways, e.g. by the Stober process or spray drying. As organic component, use is made of at least one compound which is selected from among organosilicon compounds and organic compounds of the cations of the elements specified under (a) and (b). The expression xe2x80x9corganic compoundxe2x80x9d is intended to refer to any xe2x80x9corganometallicxe2x80x9d compound which has at least one organic constituent bound or complexed to the metal via oxygen or an organic constituent bound to the metal in such a way that at least partial hydrolysis of this compound can occur in the presence of water, aqueous or other solvents or dispersion media (e.g. alcohols), which hydrolysis may possibly only start in the presence of acid or base, after which the compound undergoes a controlled condensation so that chain or network condensates are formed in the xe2x80x9csolventxe2x80x9d but no uncontrolled precipitation reactions occur (the expression xe2x80x9csolventxe2x80x9d is naturally to be interpreted in such a way that the medium generally does not form a genuine solution of the organic compound(s) but usually forms a suspension, a dispersion, an emulsion, a sol or a gel) . Examples of organic compounds are oxo complexes such as alkoxides or carboxylates, but also other suitable metal complexes or organometallic compounds. Depending on requirements, a plurality, or all, of the cations of the subsequent spherical particles can be used in the form of the abovementioned organic compounds.
Elements which can be used under (a) are preferably those of main group III, i.e. boron, aluminum, gallium, indium and thallium. Further suitable elements are scandium, yttrium and rare earths such as lanthanum, cerium, gadolinium, ytterbium. Owing to the favorable structures of aluminosilicates, aluminum is preferred. The selection of specific elements, e.g. very heavy elements, enables particular properties such as X-ray opacity to be produced.
If the element or one (or all) of the elements specified under (a) is to be used as organic component, preference is given to using oxo complexes for this purpose. Suitable oxo complexes are, for example, alkoxides, diketonates and carboxylates. Examples of alkoxides are ethoxide, secondary and tertiary butoxide, e.g. of aluminum. Examples of carboxylates are those of oxalic acid and methacrylic acid. Acetates or acetylacetonates and also further complexes with chelating agents are also possible. If instead or additionally one or the (or all) element(s) specified under (a) is/are not to be used as organic component, use in the form of possibly extremely fine powders of the corresponding inorganic compounds which may be soluble or insoluble in the solvent selected, e.g. oxides, halides (chlorides, fluorides), phosphates or other salts (e.g. AlCl3), is possible.
The metals among which the cations of the group specified under (b) can be selected encompass, for example, lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium, tin or zinc (the latter in its divalent form). The selection of suitable cations enables specific properties to be generated in a targeted way, for example X-ray opacity, reactivity, optical properties or the like.
When the element or one (or all) of the elements specified in group (b) is to be used as organic component, possible compounds are, not only but in particular, the carboxylates and alkoxides. Particular preference is given to magnesium acetate, calcium acetate and strontium acetate and the alkoxides, e.g. isopropoxides, of these elements. If instead or additionally one or the (or all) element(s) specified under (b) is/are not to be used as organic component, use in the form of possibly extremely fine powders of the corresponding inorganic compounds which may be soluble or insoluble in the solvent chosen, e.g. oxides, halides (chlorides, fluorides), phosphates or other salts (e.g. MgCl2, SnCl2), is possible.
The silicon ions required for the outer region can be incorporated into the ionomer particles in various ways. Thus, for example, hydrolyzable silanes or siloxanes, for example alkylsilanes and/or alkoxysilanes, can be added to the dispersion. In this case, particles having a homogeneous silicate-containing matrix in the outer region are obtained. Alternatively, a dispersion comprising compounds of the complexed elements specified in group (a) and/or in group (b) can, for example be admixed with a second dispersion of silicon dioxide particles having a very small diameter. In this case, the silicon dioxide forms cluster-like structures within the outer region of the resulting particles which, owing to their small diameter, are very well crosslinked with the oxides of the other elements.
Depending on the field of application, various further substances can be mixed into the abovementioned dispersion. An example is the incorporation of tin dioxide particles into- a sol comprising the abovementioned constituents. In this way, it is possible to obtain spherical particles which have an inner region of tin dioxide and, for example, achieve good X-ray absorption. The core of the ionomer particles can instead comprise silicon dioxide; for this purpose, silicon dioxide particles of the appropriate size (e.g. having a diameter of 30-100 nm (e.g. for dental purposes) or from 1 to 2 xcexcm) are brought into contact with the dispersion, so that the latter can deposit in the outer region around the silicon dioxide core. The abovementioned ionomer-reactive modifications are mentioned only by way of example; all possible variants can be employed as long as the ionomer particles have the abovementioned ionomer-reactive constituents in their outer region.
The abovementioned organically modified constituents for producing the dispersion can, for example, be introduced into water and, if desired, admixed with acetic acid or glacial acetic acid (or be introduced into the previously acidified solvent). Basic solvents are also possible. As an alternative, the organically modified components, for example in a nonaqueous dispersion medium, e.g. an alcohol, can be admixed in an appropriate way with an amount of water sufficient for the required hydrolysis processes and, if desired, with base or acid as catalyst. Beforehand or afterwards, any inorganic-substances to be processed together with the ionomer can be incorporated; these inorganic substances may, if desired, previously have been dissolved or dispersed. In this environment, hydrolytic condensation of the organically modified constituents commences, but the reaction conditions have to be regulated so that hydroxides or oxides do not precipitate in an uncontrolled manner. Rather, the constituents are converted into chains and/or a network in which the hydrogen bonds present are sufficient to obtain a stable framework throughout the space from which the particles develop (i.e. a dispersion or suspension) or throughout the entire liquid (with formation of a sol or gel).
The above-described constituents form a dispersion which, according to the invention, is converted into spherical or approximately spherical particles, or are separated off from such particles. This can be done in various ways.
The prior art discloses a series of processes which can, for example, be utilized for producing the ionomer particles of the invention.
Stxc3x6ber and Fink (Journal of Colloid and Interface Science 26, 62-69 (1968)) describe the production of agglomerate-free, monodisperse silica particles starting from tetraalkoxysilanes and water in alcohol in the presence of ammonia as catalyst (generally known as the Stober process). The particle size can be adjusted within the range from 50 to 2000 nm via various parameters such as water concentration, ammonia concentration, choice of alkoxysilane and temperature.
EP 216 278 describes a process based on the Stober process for producing monodisperse, nonporous silica particles which have a size of from 50 to 10,000 nm and whose monodispersity has been optimized to less than 5%.
On the basis of the Stober process, DE-C 3247800 describes the production of spherical, amorphous mixed particles based on silica and from 0.01 to 20 mol % of an oxide of a metal of groups I-IV of the Periodic Table.
DE 38 34 774 gives a review of the state of the art of the emulsion process for water-in-oil emulsions (W/O). Minehan and Messing (Colloids and Surfaces 63, 181 to 187 (1992)) describe the production of silica particles via oil-in-water emulsions (O/W). Microemulsion methods are known for producing nanosize particles ( less than 100 nm), for example the production of SiO2 particles in nonionic multicomponent systems, as described by K. Osseo-Asare, F. J. Arrigada, Colloids and Surfaces 50, 321-339 (1990). Particles having a size range of 50-70 nm with standard deviations of less than 8.5% are obtained.
Various solution aerosol thermolysis (SAT) processes, e.g. spray drying, are known for producing powders (G. V. Jayanthi, S. C. Zhang, G. L. Messing, Aerosol Science and Technology, 19, 478-490 (1993)). SAT represents a class of production processes in which a precursor liquid is sprayed by means of nozzles into a preheated oven. Alternatively, it is also possible to use a chopper (R. N. Berglund et al., Environ. Sci. Technol. 1973, 7 (2), 147-153). The resulting droplets are, depending on the diameter of the nozzle or the chopper frequency, in the range from less than 1 xcexcm to more than 50 xcexcm. The solvent vaporizes at the high temperatures in the oven, so that dry powders are formed.
The abovementioned processes can be utilized in principle for producing the spherical or approximately spherical ionomer particles of the invention. In place of the hydrolyzable compounds employed in the above processes, it is necessary for the organic and any other compounds which are essential in the present invention to be introduced into the liquid.
According to the invention, it is possible, for example by means of a method based on the Stxc3x6ber process, to apply a shell containing silicon ions and additional elements of groups a) and b) to various inert particle cores (e.g. SiO2, SnO2 cores). As cores, it is possible to use monodisperse, spherical nuclei produced in any way. Examples of suitable cores are commercially available, agglomerate-free, monodisperse, spherical SiO2 (e.g. Ludox, DuPont) or SnO2 particles. Using a method based on the abovementioned Stxc3x6ber process, it is also possible to produce monodisperse spherical Sio2 cores in a size range from 50 to 2000 nm which are then provided with a xe2x80x9cshellxe2x80x9d.
To apply the shell, organosilicon compounds such as, for example, alkoxysilanes in combination with compounds of groups a) and b) (the two latter in organic or inorganic form) can be used as starting compounds. The organic compounds or a low molecular weight condensation product thereof are added in an amount of, for example, 1-40% by weight to a solvent, preferably an alcohol. This solution is titrated into a mother dispersion of the cores so that a supersaturation concentration which would lead to formation of new particles is not reached during the course of the growth process of the particles. Since the organic compounds are to be hydrolyzed in this process, water is added in a concentration matched to the concentration of the starting materials. Since the hydrolysis/condensation reactions proceed very slowly under neutral conditions, an acidic or alkaline medium is preferred. For the purposes of the present invention, it has been found that a pH of 8-9 is advantageous for uniform growth of the particles and that ideally spherical, monodisperse ionomer particles result. These have a surprisingly rapid ionomer reaction.
In-situ surface modification can be achieved, for example, by addition of a silane, e.g. aminopropyltriethoxysilane or methacryloxypropyltrimethylsilane, in the form of a 1-100% strength by weight solution to the dispersion. As solvent, preference is given to using the same solvent as that of the mother dispersion, for example ethanol. It is likewise possible to surfacemodify the dried particles afterwards. For this purpose, the particle powder is suspended in an amount of about 10% by weight in an organic solvent, e.g. toluene, the amount of silane necessary to form a monomolecular layer is added, a catalyst is introduced if desired and, if desired, the mixture is refluxed.
The Stxc3x6ber process can, of course, also be used for producing homogeneous or other particles without a core. The conditions then have to be chosen so that particle formation is induced.
Furthermore, it has been found that emulsion processes are also very well suited to producing the above-described ionomer particles. It is possible to use both the O/W method and the W/O method. Preference is given to using the W/O method (see, for example, EP 0363927). The proportion of aqueous phase is preferably from about 15 to 45% by volume, while that of the emulsifier is preferably from about 1 to 20% by weight. During the emulsion process, precipitation or gel formation, preferably induced by a basic pH shift, takes place in the aqueous droplets. Suitable starting compounds are salts and organic complexes of the above-described elements, preferably nitrates, alkoxides and acetates, and also any dispersions produced therefrom. The ionomer particles obtained surprisingly have a narrow size distribution which can be significantly less than one power of ten.
The above-described liquid can instead be subjected to an aerosol treatment, in particular spray drying. For example, very finely divided SiO2 particles or silicon alkoxides can be mixed with alkoxides or carboxylates of the cations of groups a) and b) in an aqueous solution having a pH of  less than 7. Spherical-shaped droplets are sprayed by means of suitable nozzles. These droplets can, if desired, be dried, for example at about 250xc2x0 C., until the volatile organic constituents have been removed.
In all the processes, the particles obtained can, after removal of the solvent or after separation from the solvent, can, if desired, be subjected to a pyrolysis if organic constituents are still present (for example at from 400xc2x0 to 600xc2x0 C.). This produces organic-free siliceous ionomer particles.
Depending on the starting compounds used, ionomer particles of differing structure are formed in the abovementioned process. The particles can have a fully homogeneous structure or else have silicate-containing regions together with calcium aluminosilicates, strontium aluminosilicates or the like or regions comprising exclusively silicon as cations together with regions comprising calcium and/or strontium, aluminum and silicon or the like. The ionomer particles can consist exclusively of these structures or else can have a discreet inner region which has a different composition, for example silicon dioxide, tin dioxide, a mixture of the two, aluminum silicate or the like. In a specific embodiment, the spherical ionomer particles consist of an inner region and a plurality of outer, preferably shell-like, regions. These can be produced, for example, by enveloping silicon dioxide particles of suitable size with a first gel or sol, drying and, if appropriate, pyrolyzing them, and then enveloping the resulting particles with a second gel or sol having a different composition, drying and, if appropriate, pyrolyzing again. At least the second gel or sol has to have a composition as described above. The spherical ionomer particles of the invention can also be silanized or otherwise surface-modified in a customary manner, although this will generally not be necessary.
Incorporation of the ionomers of the invention into matrix systems, preferably into acid-containing matrix systems, results after the above-described two-staged curing in new types of materials, e.g. composites, cements, compomers. Their properties can, as described above, be set in a targeted way by use of appropriate starting materials, e.g. by addition of X-ray-opaque constituents or by means of reaction conditions (e.g. concentrati on, temperature, pH) which allo w the diameter of the particles to be varied. These materials are particularly usef ul in dentistry (e. g. as filling material) and in the medical sector (e.g. as bone cement). Furthermore, materials having a different transparency, color and a different index of refraction can be used.
It has surprisingly been found that the spherical shape and size of the ionomer particles of the invention lead to significantly imp roved abrasion re sistance and mechanical properties of dental fillings when the particles are processed together with alkenoic acids to produce a cement. Furthermore, it was surprisingly found that the rate of the ionomer reaction and of cement formation is greatly increased and the degree of reaction has increased. This can be confirmed by means of IR spectroscopy.
Suitable matrix systems with which the ionomer particles of the invention can be processed to give cements are the abovementioned alkenoic acids, e.g. polyalkenoic acids (monocarboxylic, dicarboxylic or tricarboxylic acids) such as polyacrylic acid, polyitaconic acid, polymaleic acid or copolymers of these acids, to which, if desired, hydroxycarboxylic acids such as citric acid or tartaric acid can be added. These can be employed in an aqueous phase or freeze-dried; in the latter state, water naturally has to be added when making up the mixture with the ionomers. However, other, preferably acidic matrix systems are also possible, e.g. polyphosphonic acids such as poly(vinylphosphonic acid), systems which further comprise light-curable constituents or matrices which can form the above-described compomers with ionomer particles.